How long should a hill be for hill repeats? The rep sets the hill

At a glance

The arithmetic: rep length times climbing speed

Hill length is rep time multiplied by your climbing speed at the target effort. The speeds in the table assume a rider near 3 W/kg on a 5 to 7 percent grade. They are assumptions, not norms: a stronger rider goes faster and needs a longer hill, a heavier or newer rider needs a shorter one.

The worked example is simple. 15 km/h is 250 m per minute, so a 4-minute rep covers 1.0 km; at 18 km/h it covers 1.2 km. Add 10 to 20 percent and you want a climb of about 1.1 to 1.4 km with no flat section in the middle. The margin matters because the first 20 to 30 seconds of a rep are slower while you build to the target, and a rep that runs out at 3:40 is a different session from the one you planned.

To find your own speed, look at an old ride. Pick a climb you rode at roughly the target effort, read its length and your time, and divide. A 1.5 km segment ridden in 5:30 is 16.4 km/h. Use that number in place of the table's assumption. The shorter the rep, the faster you go, because the power is higher: an all-out 30-second effort can run 20 to 25 km/h on a grade where threshold runs 15.

Rep length comes from the workout, and the workout comes from your plan. Seiler's group compared 4x4, 4x8 and 4x16 minutes in 35 trained recreational cyclists; 4x8 at about 90 percent of peak heart rate gave the largest gains in VO2peak and power at a lactate of 4 mmol/L over 7 weeks [Seiler et al. 2013]. Rønnestad found short 30/15-second intervals beat effort-matched long intervals over 10 weeks [Rønnestad et al. 2015]. Both shapes work. Pick the hill that fits the one you are riding today, not the session that fits the hill you happen to have.

How steep: why too steep and too shallow both fail

About 4 to 7 percent holds most interval targets. On steeper grades speed drops so far that cadence falls and power goes spiky; that is fine for 30 seconds to a minute, hard to control for 8. Under about 3 percent, air drag takes over and the hill behaves like flat road.

Gear arithmetic shows the steep-hill problem. A 75 kg rider holding 250 W on a 12 percent grade climbs at roughly 9 km/h. In a 34x28 gear on a 700x28c wheel that is about 56 rpm, and every small ramp drops it further. Riders then stand, surge and sit, and the power trace swings far outside the target range. That is our reasoning from the physics, not a measured finding, but anyone who has tried a steady 8-minute effort up a 12 percent wall knows the trace it gives.

Low cadence is not the enemy in itself. Nimmerichter's cyclists rode 6x5 minutes uphill at 60 rpm, twice a week for 4 weeks, and raised 20-minute uphill time-trial power by 4.4 percent and flat power by 1.5 percent [Nimmerichter et al. 2012]. The point is control: 60 rpm on a steady 6 percent grade is a chosen drill; 50 rpm on a 14 percent ramp is the hill choosing for you. Gross efficiency also changes with grade and cadence in field riding, so the same watts do not feel the same on every slope [Nimmerichter et al. 2015].

Steep grades do suit the shortest reps. Hovorka tested 13 trained athletes on flat (0.6 percent) and steep (9.8 percent) roads: 1-minute power and W', the work available above critical power, were higher uphill, while 4- and 10-minute power showed no significant difference [Hovorka et al. 2022]. The authors still found the limits of agreement too wide to call the two conditions equivalent for any one rider. So a steep hill is a good tool for 30-second and 1-minute repeats, and a steady moderate grade is the better tool for long reps.

Too shallow fails the other way. At 2 percent the same 250 W rider rolls at about 26 km/h, so air drag is now a large share of the load and the climbing feel mostly goes. A 4-minute rep would need about 1.8 km of 2 percent road, and wind or a slight dip changes the effort more than the grade does. Treat a drag under 3 percent as flat road and follow the pacing rules for threshold intervals without hills instead.

When the hill is shorter than the rep

Most riders have the wrong hill, not no hill. If the climb ends before the rep does, keep the same power range over the crest and onto the flat, ride a loop that strings two climbs together, or move the session to a longer climb or flat road. Do not quietly shorten the reps.

Riding over the top is the simplest fix. A 600 m climb plus 400 m of flat road can carry a 4-minute rep if you shift up at the crest and hold the same watts. The power target is the session, not the gradient. Power on a head unit makes this easy; heart rate alone lags by a minute or more and will read low just as you change terrain.

Cutting a 4x8 into 16 reps of 2 minutes on a short hill is a different workout. Seiler's data show that how long each work period lasts changes the intensity you can hold and the adaptation you get [Seiler et al. 2013]. If the only hill is short, give it the workout that fits it: 30-second to 1-minute repeats on a 300 m ramp are a real session, and a VO2max day can move to the trainer or a flat road. This is the wider idea behind doing intervals outside: pick the road for the workout, and accept that some sessions belong on a different road.

Climb databases such as PJAMM and climbfinder list length and average grade for named climbs, and old ride files show how long you took on each. Our Suggested Routes, a paid feature, sizes a loop to today's planned time from your own speed and checks for 3 to 8 percent climbs of 200 m or more near the start; it picks a loop where the work fits rather than marking where each rep starts. Gritmaps, a free beta, takes a different approach and lines climbs up with effort blocks from an intervals.icu workout. Any of these beats searching a map the night before.

The descent is your recovery, so make it safe

On a hill repeat the descent sets the recovery. A 1.2 km descent at 35 km/h takes about 2 minutes, so the hill decides your work-to-rest ratio. Plan the turnaround at both ends, descend at an easy spin, and never treat the way down as part of the effort.

Time the descent before the first rep. If the planned recovery is 3 minutes and the descent takes 2, spin easy at the bottom for the extra minute. If the descent takes 5 minutes, your recovery is 5 minutes; for threshold reps that is usually fine, for 30/15-style work it is not, because 15 seconds of rest cannot include a descent [Rønnestad et al. 2015]. Those sessions belong on a long climb with on-off efforts, on flat road, or on a trainer.

Safety is reasoning, not a study: no paper measures crash risk on hill-repeat descents. But you reach the top with heart rate near peak and focus narrowed, then turn into traffic and descend. Choose a turnaround with clear sight lines both ways, stop at the side of the road rather than U-turning on a blind bend, and avoid descents with junctions or gravel on the apex. In cool weather, a wet jersey at 40 km/h chills fast; a wind vest for the descent is cheap insurance.

Quick answers

Sources cited in this guide

1. 01
   Nimmerichter et al. 2012. Effects of low and high cadence interval training on power output in flat and uphill cycling time-trials. European Journal of Applied Physiology.
2. 02
   Nimmerichter et al. 2015. Gross efficiency during flat and uphill cycling in field conditions. International Journal of Sports Physiology and Performance.
3. 03
   Hovorka et al. 2022. Effects of Flat and Uphill Cycling on the Power-duration Relationship. International Journal of Sports Medicine.
4. 04
   Seiler et al. 2013. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine and Science in Sports.
5. 05
   Rønnestad et al. 2015. Short intervals induce superior training adaptations compared with long intervals in cyclists - an effort-matched approach. Scandinavian Journal of Medicine and Science in Sports.

More inside How to do intervals outside

Other articles in this series

1. 01

   The best road for intervals: what to look for

   Long enough for the rep, no lights, a safe turnaround. How to find a stretch of road that lets an interval session run as written.

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2. 02

   VO2 max intervals outside: hill or flat road?

   Uphill and flat VO2 reps are not the same work. What the research says about each, and how to ride 5x4 when you have no hills.

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3. 03

   Threshold intervals without hills: rolling and flat roads

   No long climb nearby? How to hold threshold and sweet spot on flat or rolling roads: pace by range, use the wind, fit reps to the road.

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4. 04

   Zone 2 outside: how to plan a ride without stops

   Lights and junctions break an easy ride into pieces. How to plan a Zone 2 loop that stays steady, and what to do when you cannot.

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